PKHD1基因缺陷与常染色体隐性遗传性多囊肾病的研究进展

PKHD1是目前所知人类常染色体隐性遗传多囊肾病(autosomal recessive polycystic kidney disease,ARPKD)的惟一致病基因。ARPKD临床病变以双肾多发性进行性充液囊泡为主要特征。目前对PKHDl基因在ARPKD发病中的作用了解并不多。该文对ARPKD的发病机制和PKHD1基因功能最新研究进展进行综述。     

Germline PKHD1 mutations are protective against colorectal cancer

The autosomal recessive polycystic kidney disease (ARPKD) gene, PKHD1, has been implicated in the genesis or growth of colorectal adenocarcinoma, as a high level of somatic mutations was found in colorectal tumor tissue. To determine whether carriers of a single PKHD1 mutation are at increased risk of colorectal carcinoma, we assessed the prevalence of the commonest European mutation, T36M. First, we assayed a European cohort of ARPKD patients and found T36M was responsible for 13.1% of mutations. We then investigated two European cohorts with colorectal adenocarcinoma versus two control cohorts of similar age and gender. Screening for the most common PKHD1 mutation, T36M, we detected 15:3,603 (0.42%) controls versus 1:3,767 (0.027%) colorectal cancer individuals, indicating that heterozygous PKHD1 mutations are not a risk factor and are protective (p=0.0002). We also show that the carriage rate for PKHD1 mutations in the European population is higher than previous accepted at 3.2% (1:31 genomes).     

Regulation of cell proliferation and apoptosis through fibrocystin-prosaposin interaction

Mutations in PKHD1 (polycystic kidney and hepatic disease gene 1) gene cause the autosomal recessive polycystic kidney disease (ARPKD). It is widely accepted that cystogenesis is owing to aberrant cell proliferation and apoptosis, increased fluid secretion, and extracellular matrix abnormality. Fibrocystin/polyductin (FPC), the encoded protein product by PKHD1, is a single transmembrane protein and believed to be a novel receptor-like molecule. FPC has been located mainly on the plasma membrane and cilium/basal body. However, its biological functions remain poorly understood. To investigate the roles of FPC in the pathogenesis of ARPKD, we searched for FPC-interacting proteins by yeast two-hybrid assay, and found a novel partner, prosaposin. Prosaposin is a glycoprotein with multiple functions. With GST pull-down assay and co-immunoprecipitation, we confirmed the interaction between FPC and prosaposin. In order to study the effects of FPC-prosaposin interaction on cell proliferation and apoptosis, we have made stable cell lines in which FPC was overexpressed or knocked down alone or in combination with prosaposin overexpression. By MTT assay, we found that FPC knockdown and prosaposin overexpression increased cell proliferation, respectively, while overexpression of FPC C-tail did the opposite. With apoptosis assay, we found that overexpression of FPC C-tail promoted cell apoptosis. However, overexpression of prosaposin significantly enhanced cell survival in FPC knockdown cells. All these findings indicated that FPC and prosaposin may play significant roles in regulation of cell proliferation and apoptosis. Taken together, we have disclosed a novel signaling pathway of FPC, which may be important for the pathogenesis of ARPKD.     

Molecular and cellular pathophysiology of autosomal recessive polycystic kidney disease (ARPKD)

Autosomal recessive polycystic kidney disease (ARPKD) belongs to a group of congenital hepatorenal fibrocystic syndromes characterized by dual renal and hepatic involvement of variable severity. Despite the wide clinical spectrum of ARPKD (MIM 263200), genetic linkage studies indicate that mutations at a single locus, PKHD1 (polycystic kidney and hepatic disease 1), located on human chromosome region 6p21.1–p12, are responsible for all phenotypes of ARPKD. Identification of cystic disease genes and their encoded proteins has provided investigators with critical tools to begin to unravel the molecular and cellular mechanisms of PKD. PKD cystic epithelia share common phenotypic abnormalities despite the different genetic mutations that underlie the disease. Recent studies have shown that many cyst-causing proteins are expressed in multimeric complexes at distinct subcellular locations within epithelia. This co-expression of cystoproteins suggests that cyst formation, regardless of the underlying disease gene, results from perturbations in convergent and/or integrated signal transduction pathways. To date, no specific therapies are in clinical use for ameliorating cyst growth in ARPKD. However, studies noted in this review suggest that therapeutic targeting of the cAMP and epidermal growth factor receptor (EGFR)-axis abnormalities in cystic epithelia may translate into effective therapies for ARPKD and, by analogy, autosomal dominant polycystic kidney disease (ADPKD). A particularly promising approach appears to be the targeting of downstream intermediates of both the cAMP and EGFR axis. This review focuses on ARPKD and presents a concise summary of the current understanding of the molecular genetics and cellular pathophysiology of this disease. It also highlights phenotypic and mechanistic similarities between ARPKD and ADPKD.The authors are supported by the National Institutes of Health (grant no. 1-P50-DK57306), the PKD Foundation (grant no. 76a2r), and the Children’s Research Institute, Children’s Hospital of Wisconsin.     

Down-regulation of PKHD1 induces cell apoptosis through PI3K and NF-κB pathways

Mutations in PKHD1 (polycystic kidney and hepatic disease gene 1) gene cause the autosomal recessive polycystic kidney disease (ARPKD). Fibrocystin/polyductin (FPC), encoded by PKHD1, is a membrane-associated receptor-like protein. Although it is widely accepted that cystogenesis is mostly due to aberrant cell proliferation and apoptosis, it is still unclear how apoptosis is regulated. The aim of this study is to analyze the relationship among apoptosis, phosphatidylinositol 3-kinase (PI3K)/Akt and nuclear factor κB (NF-κB) in FPC knockdown kidney cells. We show that PKHD1-silenced HEK293 cells demonstrate a higher PI3K/Akt activity. Selective inhibition of PI3K/Akt using LY294002 or wortmannin in these cells increases serum starvation-induced HEK293 cell apoptosis with a concomitant decrease in cell proliferation and higher caspase-3 activity. PI3K/Akt inhibition also leads to increased NF-κB activity in these cells. We conclude that the PI3K/Akt pathway is involved in apoptotic function in PKHD1-silenced cells, and PI3K/Akt inhibition correlates with upregulation of NF-κB activity. These observations provide a potential platform for determining FPC function and therapeutic investigation of ARPKD.     

Comparative proteomic analysis suggests that mitochondria are involved in autosomal recessive polycystic kidney disease

Autosomal recessive polycystic kidney disease (ARPKD), characterized by ectatic collecting duct, is an infantile form of PKD occurring in 1 in 20 000 births. Despite having been studied for many years, little is known about the underlying mechanisms. In the current study, we employed, for the first time, a MS-based comparative proteomics approach to investigate the differently expressed proteins between kidney tissue samples of four ARPKD and five control individuals. Thirty two differently expressed proteins were identified and six of the identified protein encoding genes performed on an independent group (three ARPKD subjects, four control subjects) were verified by semi-quantitative RT-PCR, and part of them were further validated by Western blot and immunohistochemistry. Moreover, similar alteration tendency was detected after downregulation of PKHD1 by small interfering RNA in HEK293T cell. Interestingly, most of the identified proteins are associated with mitochondria. This implies that mitochondria may be implicated in ARPKD. Furthermore, the String software was utilized to investigate the biological association network, which is based on known and predicted protein interactions. In conclusion, our findings depicted a global understanding of ARPKD progression and provided a promising resource of targeting protein, and shed some light further investigation of ARPKD.     

A novel mutation identified in PKHD1 by targeted exome sequencing: Guiding prenatal diagnosis for an ARPKD family

Autosomal recessive polycystic kidney disease (ARPKD) is a rare hereditary renal cystic disease involving multiple organs, mainly the kidney and liver. Parents who had an affected child with ARPKD are in strong demand for an early and reliable prenatal diagnosis to guide the future pregnancies. Here we provide an example of prenatal diagnosis of an ARPKD family where traditional antenatal ultrasound examinations failed to produce conclusive results till 26th week of gestation. Compound heterozygous mutations c.274C>T (p.Arg92Trp) and c.9059T>C (p.Leu3020Pro) were identified using targeted exome sequencing in the patient and confirmed by Sanger sequencing. Further, the mother and father were revealed to be carriers of heterozygous c.274C>T and c.9059T>C mutations, respectively. Molecular prenatal diagnosis was performed for the current pregnancy by direct sequencing plus linkage analysis. Two mutations identified in the patient were both found in the fetus. In conclusion, compound heterozygous PKHD1 mutations were elucidated to be the molecular basis of the patient with ARPKD. The newly identified c.9059T>C mutation in the patient expands mutation spectrum in PKHD1 gene. For those ultrasound failed to provide clear diagnosis, we propose the new prenatal diagnosis procedure: first, screening underlying mutations in PKHD1 gene in the proband by targeted exome sequencing; then detecting causative mutations by direct sequencing in the fetal DNA and confirming results by linkage analysis.     

Inhibition of Pkhd1 impairs tubulomorphogenesis of cultured IMCD cells

Fibrocystin/polyductin (FPC), the gene product of PKHD1, is responsible for autosomal recessive polycystic kidney disease (ARPKD). This disease is characterized by symmetrically large kidneys with ectasia of collecting ducts. In the kidney, FPC predominantly localizes to the apical domain of tubule cells, where it associates with the basal bodies/primary cilia; however, the functional role of this protein is still unknown. In this study, we established stable IMCD (mouse inner medullary collecting duct) cell lines, in which FPC was silenced by short hairpin RNA inhibition (shRNA). We showed that inhibition of FPC disrupted tubulomorphogenesis of IMCD cells grown in three-dimensional cultures. Pkhd1-silenced cells developed abnormalities in cell-cell contact, actin cytoskeleton organization, cell-ECM interactions, cell proliferation, and apoptosis, which may be mediated by dysregulation of extracellular-regulated kinase (ERK) and focal adhesion kinase (FAK) signaling. These alterations in cell function in vitro may explain the characteristics of ARPKD phenotypes in vivo.     

Zystennieren – eine Übersicht

Cystic kidney diseases are a clinically and genetically heterogeneous group of disorders, representing one of the most frequent genetic conditions with a prevalence of about 1 in 1000. The most important forms include autosomal dominant polycystic kidney disease (ADPKD) caused by mutations in the PKD1 and PKD2 genes and the autosomal recessive polycystic kidney disease (ARPKD) caused by mutations in the PKHD1 gene. The proteins encoded by the involved genes are summarized as cystoproteins. On the cellular level, the majority of these cystoproteins co-localize in primary cilia, the basal body or the centrosome of renal epithelial cells. Inherited polycystic kidney diseases belong to the increasing number of reported ciliopathies which include many syndromic forms, e.g. Bardet-Biedl syndrome, Meckel syndrome and Joubert syndrome. Identifying the genetic defect can help establish the correct diagnosis, define the clinical prognosis and forms the basis for genetic counselling. In addition to establishing a clinical, ultrasonographic and morphological picture of the underlying kidney disease, the algorithm of genetic diagnosis should take the presence of further organ dysfunction or malformation as well as family history into consideration.     

Nek8 mutation causes overexpression of galectin-1, sorcin, and vimentin and accumulation of the major urinary protein in renal cysts of jck mice

The jck murine model, which results from a double point mutation in the nek8 gene, has been used to study the mechanism of autosomal recessive polycystic kidney disease (ARPKD). The renal proteome of jck mice was characterized by two-dimensional gel electrophoresis combined with mass spectrometry (MALDI-TOF/TOF). Four newly identified proteins were found to accumulate in the kidneys of jck mice with polycystic kidney disease (PKD) compared with their wild-type littermates. The proteins galectin-1, sorcin, and vimentin were found to be induced 9-, 9-, and 25-fold, respectively, in the PKD proteome relative to the wild type. The identity of these proteins was established by peptide mass fingerprinting and de novo MS/MS sequencing of selected peptides. Up-regulation of these three proteins may be due to the nek8 mutation, and their function may be related to the signaling and structural processes in the primary cilium. Additionally a series of protein isoforms observed only in the ARPKD kidney was identified as the major urinary protein (MUP). Peptide sequencing demonstrated that the isoforms MUP1, MUP2, and MUP6 are contained in this series. The MUP series showed a number of male-specific isoforms and a phosphorylation of the entire series with an increasing degree of phosphorylation of the acidic isoforms. In addition, the MUP series was localized to the cyst fluid of PKD mice, and a cellular mislocalization of galectin-1, sorcin, and vimentin in PKD tubular epithelial cells was shown. The abnormal and extremely high accumulation of the MUPs in the ARPKD kidney may be linked to a defect in protein transport and secretion. The discovery of these proteins will provide new information on the molecular and cellular processes associated with the mechanism of ARPKD.     

Atmin modulates Pkhd1 expression and may mediate Autosomal Recessive Polycystic Kidney Disease (ARPKD) through altered non-canonical Wnt/Planar Cell Polarity (PCP) signalling

Autosomal Recessive Polycystic Kidney Disease (ARPKD) is a genetic disorder with an incidence of ~1:20,000 that manifests in a wide range of renal and liver disease severity in human patients and can lead to perinatal mortality. ARPKD is caused by mutations in PKHD1, which encodes the large membrane protein, Fibrocystin, required for normal branching morphogenesis of the ureteric bud during embryonic renal development. The variation in ARPKD phenotype suggests that in addition to PKHD1 mutations, other genes may play a role, acting as modifiers of disease severity. One such pathway involves non-canonical Wnt/Planar Cell Polarity (PCP) signalling that has been associated with other cystic kidney diseases, but has not been investigated in ARPKD. Analysis of the Atmin^Gpg6 mouse showed kidney, liver and lung abnormalities, suggesting it as a novel mouse tool for the study of ARPKD. Further, modulation of Atmin affected Pkhd1 mRNA levels, altered non-canonical Wnt/PCP signalling and impacted cellular proliferation and adhesion, although Atmin does not bind directly to the C-terminus of Fibrocystin. Differences in ATMIN and VANGL2 expression were observed between normal human paediatric kidneys and age-matched ARPKD kidneys. Significant increases in ATMIN, WNT5A, VANGL2 and SCRIBBLE were seen in human ARPKD versus normal kidneys; no substantial differences were seen in DAAM2 or NPHP2. A striking increase in E-cadherin was also detected in ARPKD kidneys. This work indicates a novel role for non-canonical Wnt/PCP signalling in ARPKD and suggests ATMIN as a modulator of PKHD1.     

Autosomal recessive polycystic kidney disease does not map to the second gene locus for autosomal dominant polycystic kidney disease on chromosome 4

Linkage analysis in 19 families with autosomal recessive polycystic kidney disease (ARPKD) has shown that ARPKD is not linked to the recently assigned second gene locus for autosomal dominant polycystic kidney disease (ADPKD) on chromosome 4q (PKD2). Thus, there is strong evidence that ADPKD and ARPKD have different gene loci.